[Clayart] Clayart Digest, Vol 74, Issue 48
claywork at flying-snail.com
Mon Jan 24 01:42:44 UTC 2022
> On Jan 22, 2022, at 5:44 PM, <vpitelka at dtccom.net> <vpitelka at dtccom.net> wrote:
> ...flipping the slab is not the issue. Cross rolling the slab is essential to eliminate the linear grain structure and cause the slab to shrink equally in all directions in drying and firing…
I agree with the usefulness of flipping a slab 90 degrees! The more plastic the clay, the more essential this is, IMO. When using a rolling pin, flipping to the opposite side as well as turning 90 degrees also helps loosen up the clay, making rolling easier.
My own practice is a little offbeat in this respect, though. I use a clay body with only about 5% shrinkage, so the potential warping is accordingly minimized. So, I can get away without cross-rolling for that reason, and because my forms are not overly vulnerable to warping, either…my slabs usually become box-like forms, or lately, I-beam inspired shapes, and the perpendicular elements restrain the tendency to warp. I often use long slabs - up to 40” long, sometimes, but only 12” or less in width, so even if they were inclined to warp, the long direction would be more vulnerable than the short one.
As for flipping, as opposed to rotating…it is my observation when using the Bailey single-roller design, that the bottom clay remains stuck to the canvas where it is laid down, while the upper surface gets squished and moved more, due to the way the roller indents into the starting clay lump. Even as the slab gets extended beyond the footprint of the original lump, the bottom seems to get more compressed while the top is more stretched, at least by comparison with each other. I am not much concerned with warping, but as it is a very ‘open’ body, I would rather not open it up further. Although quantitative testing is difficult, the visible grain of the clay seems more open, and it seems to ‘drink’ a bit more glaze on the ‘up’ side.
To roll a large slab, it is necessary to start with overlapping ‘pancakes’ of clay, not a giant lump taller than the roller axle height. Even when scored to lock together, the push of the roller sometimes separates these pancakes. Flipping the slab (between canvases) allows for inspection of the other side. My principal evidence of uneven rolling is that the bottom side - stuck in place to the canvas - is far more likely to show evidence of joints than the top surface which got squished bu the roller’s curvature. Even if flipping were not a good way to equalize this, it permits inspection of the underside and remediation of any fissures. With any slab roller, I usually roll halfway to the final thickness in the first pass, then roll to the final thickness in the second pass, but with a single-roller, I invert the clay and reverse its direction, too.
I suspect, but have not adequately confirmed as proof, whether a more plastic clay tends to curl toward the roller-side if rolled in one pass with a single-roller device (or rolling pin), and not rotated for a final roll. It's hard to be sure… does a slab laid on the table rolled-side-up curl on the ends more than one laid roller-side-down? Or is it just drying more unevenly in that location, or on that day? Or does cross-rolling or ribbing the surface tend to negate the effect of the single pass, single roller? Without a rigorous comparison controlled for variables, it's hard to distinguish causes. When slabs get cut and assembled before drying very much, it also reduces the visible data. This possibility would be a particular issue with slabs too long to be rolled 90 degrees to the previous pass due to device width…it would be less of a concern with a rolling pin, of course, and If I were inclined to use highly plastic clay for large, long slabs, I would probably cross-roll them with the rolling pin after doing the rolls for length with the machine.
The Brent slab roller where I taught until recently is only 12” wide, and it limited cross-rolling for that reason. I usually suggested finishing such slabs with a rolling pin, both crosswise and flipped. My own roller is 24”, but my clay seems pretty impervious to warping so I don’t bother, though I do usually rib it smooth.
I am not denying the excellent reasons for cross-rolling; only contending that flipping also has apparent value, especially with longer slabs made with a single-side roller.
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